Semiconductor devices are manufactured through several processes including a process of polishing a dielectric film, e.g., SiO2, and a process of polishing a metal film, e.g., copper or tungsten. Manufacturing processes of backside illumination CMOS sensor and through-silicon via (TSV) include a process of polishing a silicon layer (silicon wafer), in addition to the polishing processes of the dielectric film and the metal film. Polishing of the wafer is terminated when a thickness of a film (e.g., the dielectric film, the metal film, or the silicon layer), constituting a wafer surface, has reached a predetermined target value.
Polishing of a wafer is performed using a polishing apparatus. FIG. 13 is a schematic view showing an example of the polishing apparatus. The polishing apparatus typically includes a rotatable polishing table 202 for supporting a polishing pad 201, a polishing head 205 for pressing a wafer W against the polishing pad 201 on the polishing table 202, a polishing-liquid supply nozzle 206 for supplying a polishing liquid (or slurry) onto the polishing pad 201, and a film-thickness measuring device 210 for measuring a film thickness of the wafer W.
The film-thickness measuring device 210 shown in FIG. 13 is an optical film-thickness measuring device. This film-thickness measuring device 210 includes a light source 212 for emitting light, an illuminating optical fiber 215 coupled to the light source 212, a first optical fiber 216 and a second optical fiber 217 having distal ends disposed at different locations in the polishing table 202, a first optical-path switching device 220 for selectively coupling one of the first optical fiber 216 and the second optical fiber 217 to the illuminating optical fiber 215, a spectrometer 222 for measuring intensity of reflected light from the wafer W, a light-receiving optical fiber 224 coupled to the spectrometer 222, a third optical fiber 227 and a fourth optical fiber 228 having distal ends disposed at the different locations in the polishing table 202, and a second optical-path switching device 230 for selectively coupling one of the third optical fiber 227 and the fourth optical fiber 228 to the light-receiving optical fiber 224.
The distal end of the first optical fiber 216 and the distal end of the third optical fiber 227 constitute a first optical sensor 234, while the distal end of the second optical fiber 217 and the distal end of the fourth optical fiber 228 constitute a second optical sensor 235. The first optical sensor 234 and the second optical sensor 235 are arranged at different locations in the polishing table 202. As the polishing table 202 rotates, the first optical sensor 234 and the second optical sensor 235 move across the wafer W alternately. The first optical sensor 234 and the second optical sensor 235 direct the light to the wafer W, and receive the reflected light from the wafer W. The reflected light is transmitted through the third optical fiber 227 or the fourth optical fiber 228 to the light-receiving optical fiber 224, and is further transmitted through the light-receiving optical fiber 224 to the spectrometer 222. This spectrometer 222 breaks up the reflected light in accordance with wavelength and measures the intensity of the reflected light at each of wavelengths. A processor 240 is coupled to the spectrometer 222. This processor 240 generates a spectral waveform (or spectrum) from measured values of the intensity of the reflected light, and determines the film thickness of the wafer W from the spectral waveform.
FIG. 14 is a schematic view of the first optical-path switching device 220. The first optical-path switching device 220 has a piezoelectric actuator 244 for moving the distal ends of the first optical fiber 216 and the second optical fiber 217. When the piezoelectric actuator 244 moves the distal ends of the first optical fiber 216 and the second optical fiber 217, one of the first optical fiber 216 and the second optical fiber 217 is coupled to the illuminating optical fiber 215. Although not shown in the drawing, the second optical-path switching device 230 has the same structure.
The first optical-path switching device 220 and the second optical-path switching device 230 are configured to couple the first optical fiber 216 and the third optical fiber 227 to the illuminating optical fiber 215 and the light-receiving optical fiber 224, respectively, while the first optical sensor 234 is moving across the wafer W, and are further configured to couple the second optical fiber 217 and the fourth optical fiber 228 to the illuminating optical fiber 215 and the light-receiving optical fiber 224, respectively, while the second optical sensor 235 is moving across the wafer W. In this manner, the first optical-path switching device 220 and the second optical-path switching device 230 operate while the polishing table 202 is making one revolution. Therefore, the spectrometer 222 can separately process the reflected light received by the first optical sensor 234 and the reflected light received by the second optical sensor 235.
However, since the first optical-path switching device 220 and the second optical-path switching device 230 are mechanical switching devices, a malfunction may occur as a result of a long-time use. The occurrence of the malfunction in the first optical-path switching device 220 or the second optical-path switching device 230 may cause a change in the intensity of the reflected light transmitted from the first optical sensor 234 and the second optical sensor 235 to the spectrometer 222. As a result, the film thickness determined by the processor 240 may vary.